Metathesis reactions are used widely in chemical syntheses, e.g. in the form of ring-closing metathesis (RCM), cross metathesis (CM) or ring-opening metathesis (ROMP). Metathesis reactions are employed, for example, for the synthesis of olefins, for the depolymerization of unsaturated polymers and for the synthesis of telechelic polymers.
Metathesis catalysts are known, inter alia, from WO-A-96/04289 and WO-A-97/06185. They have the following in-principle structure:
where M is osmium or ruthenium, the radicals R are identical or different organic radicals having a great structural variety, X1 and X2 are anionic ligands and the ligands L are uncharged electron-donors. In the literature, the term “anionic ligands” in the context of such metathesis catalysts always refers to ligands which, when they are viewed separately from the metal centre, are negatively charged for a closed electron shell.
Recently, metathesis reactions have become increasingly important for the degradation of nitrile rubbers.
For the purposes of the present invention, a nitrile rubber, referred to as “NBR” for short, is a nitrile rubber which is a copolymer or terpolymer of at least one α,β-unsaturated nitrite, at least one conjugated diene and, if appropriate, one or more further copolymerizable monomers.
Hydrogenated nitrite rubber, referred to as “HNBR” for short, is produced by hydrogenation of nitrile rubber. Accordingly, the C═C double bonds of the copolymerized diene units in HNBR are completely or partly hydrogenated. The degree of hydrogenation of the copolymerized diene units is usually in the range from 50 to 100%.
Hydrogenated nitrite rubber is a specialty rubber which displays very good heat resistance, excellent resistance to ozone and chemicals and excellent oil resistance.
The abovementioned physical and chemical properties of HNBR are combined with very good mechanical properties, in particular a high abrasion resistance. For this reason, HNBR has found widespread use in a wide variety of applications. HNBR is used, for example, for seals, hoses, belts and damping elements in the automobile sector, also for stators, oil well seals and valve seals in the field of crude oil production and also for numerous parts in the aircraft industry, the electronics industry, machine construction and shipbuilding.
HNBR grades which are commercially available on the market usually have a Mooney viscosity (ML 1+4 at 100° C.) in the range from 55 to 120, which corresponds to a number average molecular weight Mn (determination method: gel permeation chromatography (GPC) against polystyrene standards) in the range from about 200 000 to 700 000. The polydispersity indices PDI measured (PDI=Mw/Mn, where Mw is the weight average molecular weight and Mn is the number average molecular weight), which give information about the width of the molecular weight distribution, are frequently 3 or above. The residual double bond content is usually in the range from 1 to 18% (determined by means of IR spectroscopy).
The processability of HNBR is subject to severe restrictions because of the relatively high Mooney viscosity. For many applications an HNBR grade which has a lower molecular weight and thus a lower Mooney viscosity would be desirable. This would significantly improve the processability.
Numerous attempts have been made in the past to shorten the chain length of HNBR by degradation. For example, a decrease in the molecular weight can be achieved by thermomechanical treatment (mastication), e.g. on a roll mill or in a screw apparatus (EP-A-0 419 952). However, this thermomechanical degradation has the disadvantage that function groups such as hydroxyl, keto, carboxylic acid and carboxylic ester groups are introduced into the molecule by partial oxidation and, in addition, the microstructure of the polymer is altered substantially.
For a long time, it has not been possible to produce HNBR having a low molar mass corresponding to a Mooney viscosity (ML 1+4 at 100° C.) in the range below 55 or a number average molecular weight of about Mn<200 000 g/mol by means of established production processes since, firstly, a step increase in the Mooney viscosity occurs in the hydrogenation of NBR and secondly the molar mass of the NBR feedstock to be used for the hydrogenation cannot be reduced at will since otherwise work-up in the industrial plants available is no longer possible because the rubber is too sticky. The lowest Mooney viscosity of an NBR feedstock which can be worked up without difficulties in an established industrial plant is about 30 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity of the hydrogenated nitrile rubber obtained using such an NBR feedstock is in the order of 55 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity is determined in accordance with ASTM standard D 1646.
In the more recent prior art, this problem is solved by reducing the molecular weight of the nitrile rubber before hydrogenation by degradation to a Mooney viscosity (ML 1+4 at 100° C.) of less than 30 Mooney units or a number average molecular weight of Mn<70 000 g/mol. The reduction in the molecular weight is achieved by metathesis in which low molecular weight 1-olefins are usually added. The metathesis of nitrile rubber is described, for example, in WO-A-02/100905, WO-A-02/100941 and WO-A-03/002613. The metathesis reaction is advantageously carried out in the same solvent as the hydrogenation reaction so that the degraded nitrile rubber does not have to be isolated from the solvent after the degradation reaction is complete before it is subjected to the subsequent hydrogenation. The metathesis degradation reaction is catalyzed using metathesis catalysts which are tolerant to polar groups, in particular nitrile groups.
WO-A-02/100905 and WO-A-02/100941 describe a process comprising the degradation of nitrile rubber starting polymers by olefin metathesis and subsequent hydrogenation to give HNBR having a low Mooney viscosity. Here, a nitrile rubber is reacted in the presence of a coolefin and specific complex catalysts based on osmium, ruthenium, molybdenum or tungsten in a first step and hydrogenated in a second step. In this way, it is possible to obtain hydrogenated nitrile rubbers having a weight average molecular weight (Mw) in the range from 30 000 to 250 000, a Mooney viscosity (ML 1+4 at 100° C.) in the range from 3 to 50 and a polydispersity index PDI of less than 2.5.
The metathesis of nitrile rubber can, for example, be carried using the catalyst bis(tricyclohexylphosphine)benzylideneruthenium dichloride shown below.

As a result of metathesis and hydrogenation, the nitrile rubbers have a lower molecular weight and a narrower molecular weight distribution than the hydrogenated nitrile rubbers which have hitherto been able to be produced according to the prior art.
However, the amounts of Grubbs (I) catalyst employed for carrying out the metathesis are large. In the experiments in WO-A-03/002613, they are for example, 307 ppm and 61 ppm of Ru based on the nitrile rubber used. The reaction times necessary are also long and the molecular weights after degradation are still relatively high (see Example 3 of WO-A-03/002613 where Mw=180 000 g/mol and Mn=71 000 g/mol).
US 2004/0127647 A1 describes blends based on low molecular weight HNBR rubbers having a bimodal or multimodal molecular weight distribution and also vulcanizates of these rubbers. According to the examples, 0.5 phr of Grubbs (1) catalyst is used for carrying out the metathesis. This corresponds to the large amount of 614 ppm of ruthenium based on the nitrite rubber used.
Furthermore, a group of catalysts referred to by those skilled in the art as “Grubbs (II) catalysts” is known from WO-A-00/71554.
If a “Grubbs (II) catalyst” of this type, e.g. the catalyst 1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidenylidene)tricyclohexylphosphine)(phenylmethylene)ruthenium dichloride shown below, is used for the metathesis of NBR (US-A-2004/0132891), this is successful even without use of a coolefin.

After the subsequent hydrogenation, which is preferably carried out in the same solvent, the hydrogenated nitrile rubber has lower molecular weights and a narrower molecular weight distribution (PDI) than when catalysts of the Grubbs (I) type are used. In terms of the molecular weight and the molecular weight distribution, the metathetic degradation using catalysts of the Grubbs (II) type proceeds more efficiently than when catalysts of the Grubbs (I) type are used. However, the amounts of ruthenium necessary for this efficient metathetic degradation are still relatively high. Even when the metathesis is carried out using the Grubbs (II) catalyst, long reaction times are still required.
In all the abovementioned processes for the metathetic degradation of nitrile rubber, relatively large amounts of catalyst have to be used and long reaction times are required to produce the desired low molecular weight nitrile rubbers by means of metathesis.
Even in other types of metathesis reactions, the activity of the catalysts used is of critical importance.
In J. Am. Chem. Soc. 1997, 119, 3887-3897, it is stated that in the ring-closing metathesis of diethyl diallylmalonate show below
the activity of the catalysts of the Grubbs (I) type can be increased by additions of CuCl and CuCl2. This increase in activity is explained by a shift in the dissociation equilibrium due to a phosphane ligand which leaves its coordination position being scavenged by copper ions to form copper-phosphane complexes.
However, this increase in activity brought about by copper salts in the abovementioned ring-closing metathesis cannot be applied at will to other types of metathesis reactions. Studies by the inventors have shown that, unexpectedly, although the addition of copper salts leads to an initial acceleration of the metathesis reaction in the metathetic degradation of nitrile rubbers, a significant worsening of the metathesis efficiency is observed. The molecular weights of the degraded nitrile rubbers which can be achieved in the end are substantially higher than when the metathesis reaction is carried out in the presence of the same catalyst but in the absence of the copper salts.
An as yet unpublished German patent application describes catalyst systems for metathesis in which one or more salts are used in addition to the actual metathesis catalyst. This combination of one or more salts with the metathesis catalysts leads to an increase in the activity of the catalyst. Many meanings which can be selected from various lists are in each case possible for the anions and cations of these salts. In the examples of this German patent application, the use of lithium bromide is found to be particularly advantageous both for the metathetic degradation of rubbers, e.g. nitrile rubbers, and for the ring-closing metathesis of diethyl diallylmalonate. Catalysts used here are the Grubbs (II) catalyst, the Hoveyda catalyst, the Buchmeiser-Nuyken catalyst and the Grela catalyst.
Owing to the corrosion-promoting action of bromide ions, the use of lithium bromide and also of caesium bromide is not advisable without restrictions for all metathesis reactions. In the production of low molecular weight hydrogenated nitrile rubbers, additional safety aspects, for example, play a role since after the metathetic degradation of the nitrile rubber a hydrogenation is carried out under superatmospheric pressure in steel reactors. Since water is introduced into the reaction mixture via the residual moisture content of the nitrile rubber, when the hydrogenation is carried out in the presence of lithium bromide it is necessary to ensure by means of additional measures that “pit corrosion” of the steel autoclave does not occur. For this reason, the use of bromide additions in the production of particularly low molecular weight nitrile rubbers is not an economically optimal procedure.
The examples of the abovementioned German patent application also make it obvious that the activity-increasing effect of lithium chloride is weaker than that of lithium bromide.
The increase in the activity of metathesis catalysts as a result of the addition of salts was likewise examined in Inorganica Chimica Acta 359 (2006) 2910-2917. The influences of tin chloride, tin bromide, tin iodide, iron(II) chloride, iron(II) bromide, iron(III) chloride, cerium(III) chloride*7H2O, ytterbium(III) chloride, antimony trichloride, gallium dichloride and aluminium trichloride on the self-metathesis of 1-octene to form 7-tetradecene and ethylene were studied. When the Grubbs-I catalyst was used, a significant improvement in the to conversion of 7-tetradecene was observed on addition of tin chloride or tin bromide (Table 1). Without the addition of a salt, a conversion of 25.8% was achieved, when SnCl2*2H2O was added the conversion rose to 68.5% and when tin bromide was added it rose to 71.9%. Addition of tin iodide significantly reduced the conversion from 25.8% to 4.1%. However, in combination with the Grubbs II catalyst, all three tin salts lead to only slight improvements in conversion from 76.3% (reference experiment without addition) to 78.1% (SnCl2), to 79.5% (SnBr2) and 77.6% (SnI2). When the “Phobcats” [Ru(phobCy)2Cl2 (=ChPh)] is used, the conversion is reduced from 87.9% to 80.8% by addition of SuCl2, to 81.6% by addition of SnBr2 and to 73.9% by addition of SnI2. When iron(II) salts are used in combination with the Grubbs I catalyst, the increase in conversion when iron(II) bromide is used is higher than when iron(II) chloride is used. It may be noted that regardless of the type of catalyst used, the conversion is always higher when bromides are used than when the corresponding chlorides are used.
However, the use of the tin bromide or iron(II) bromide described in Inorganica Chimica Acta 359 (2006) 2910-2917 is not an optimal solution for the preparation of nitrite rubbers because of the corrosive nature of the bromides, as described above.
In the preparation of hydrogenated nitrile rubbers, the solvent is usually removed by steam distillation after the hydrogenation. If tin salts are used as part of the catalyst system, certain amounts of these tin salts get into the wastewater which therefore has to be purified, which costs money. For this reason, the use of tin salts for increasing the activity of catalysts in the preparation of nitrite rubbers is not economically advisable.
The use of iron salts is restricted by the fact that they reduce the capacity of some ion-exchange resins which are usually used for recovering the noble metal compounds used in the hydrogenation. This likewise impairs the economics of the overall process.
Furthermore, ChemBioChem 2003, 4, 1229-1231, describes the synthesis of polymers by ring-opening metathesis polymerization (ROMP) of norbornyl oligopeptides in the presence of a ruthenium-carbene complex Cl2(PCy3)2Ru═CHphenyl, with LiCl being added. The addition of LiCl is undertaken with the declared aim of avoiding aggregation and increasing the solubility of the growing polymer chains. Nothing is reported about an activity-increasing effect of the salt addition on the catalyst.
J. Org. Chem. 2003, 68, 202-2023, too, discloses carrying out a ring-opening polymerization of oligopeptide-substituted norbornenes, in which LiCl is added. Here too, the influence of LiCl as solubility-increasing additive for the peptides in nonpolar organic solvents is emphasized. For this reason, an increase in the degree of polymerization DP can be achieved by addition of LiCl.
In J. Am. Chem. Soc. 1997, 119, 3887-3897, it is stated that metathesis catalysts containing NHC ligands, e.g. the Grubbs (II) catalyst, are treated with LiBr or NaI to replace the chloride ligands of the Grubbs (II) catalyst by bromide or iodide.
In J. Am. Chem. Soc. 1997, 119, 9130, it is stated that an improvement in yield can be achieved in the ring-closing metathesis of 1,ω-dienes by addition of tetraisopropoxytitanate so as to increase the activity of the Grubbs (I) catalyst. In the cyclization of the 9-decenoic ester of 4-pentenoate, a higher yield of the macrolide is achieved when tetraisopropoxytitanate is added than when LiBr is added. There is no indication of the extent to which this effect can also be applied to other types of metathesis reactions or other metathesis catalysts.
In Biomol. Chem. 2005, 3, 4139, the cross metathesis (CM) of acrylonitrile with itself and with other functionalized olefins using [1,3-bis(2,6-dimethylphenyl)-4,5-dihydroimidazol-2-ylidene](C5H5N)2(Cl)2Ru═CHPh is examined. The addition of tetraisopropoxytitanate improves the yield of the respective product. This publication gives the impression that the activity-increasing action of tetraisopropoxytitanate occurs only when a specific catalyst having pyridine ligands is used. There is no indication that tetraisopropoxytitanate has an effect when pyridine-free catalysts are used or in other types of metathesis reactions.
It is known from Synlett 2005, No 4, 670-672, that the addition of tetraisopropoxytitanate in the cross metathesis of allyl carbamate with methylacrylate has a negative influence on the product yield when the Hoveyda catalyst is used as catalyst. Thus, the product yield is reduced from 28% to 0% by addition of tetraisopropoxytitanate. An addition of dimethylaluminum chloride also reduces the yield from 28% to 20%. In contrast, the product yield is improved by additions of boric acid derivatives.
It is clear from the above that no teachings which indicate how the reduction of the molecular weight of nitrile rubber by metathesis can be improved can be derived from the literature since the transferability of results from one metathesis reaction to another is not apparent. The transfer of results obtained using a specific metathesis catalyst to another is also not possible.
It is therefore an object of the invention to achieve an increase in the activity of the metathesis catalyst used for reducing the molecular weight of nitrile rubber by metathesis and at the same time ensure that gelling of the nitrite rubber does not occur.